US2900569A - Photoconductive type pickup tubes - Google Patents

Description

Aug. 1s, 1959 A. D. COPE 2,900,569

PHOTOCONDUCTIVE TYPE PICKUP TUBES i Filed July l1, 1955 INVENTOR.

Unite Appleton D. Cope, Hightstown, NJ., 'assigner to Radio Corporation of America, a corporation of Delaware Application July 11, 1955, Serial No. 521,172

7 Claims. (Cl. 313-65) This invention relates to transducing electron tubes, and more especially it relates to such tubes of the electrooptical kind for converting optical images into corresponding electric signals.

A principal object is to provide an improved electron tube of the kind employing a light-responsive variable conductivity target electrode for translating light values at elemental areas of the target into corresponding electric signals.

Another object is to provide an improved photoconductive target electrode for stabilizing the operating characteristics of television pickup, or camera tubes.

A feature of the invention relates to an improved target electrode for electron tubes wherein the target includes an element which is normally of high electrical insulation when not exposed to light, but which is capable of assuming a potential at each of its elemental areas to correspond with the light intensities incident thereon, and is also capable of being restored to a predetermined datum or base potential at each area when scanned by an electron beam.

Another feature relates to an electro-optical transducing target of the light-responsive variably conductive kind arranged to be scanned over a predetermined scanning region, or raster, which may be of different shape and size from the shape and size of the layer of photoconductive material, in conjunction with a conductive backing or signal plate whose shape and size are closely correlated with the shape and size of the raster, thereby avoiding certain instabilities and non-uniformities of operation which may occur with conventional targets of the photoconductve type.

A further feature relates to a target electrode for pickup tubes and the like, wherein there is employed a composite transducing element including a highly conductive lighttransparent backing or signal plate supported on one end of the tube or face plate thereof without entirely covering the face plate; and the face plate is covered with a layer of photoconductive material which extends substantially beyond the edges of the signal plate, the size of the signal plate and its shape being closely correlated with the size and shape of the scanning raster.

VA still further feature relates to the novel organization, arrangement and relative location and dimensioning of parts, which together constitute an improved target or pickup tubes of the charge storage type.

Other features and advantages will be apparent from the ensuing descriptions and the appended claims.

In the drawing, which shows a typical example of one embodiment,

Fig. l is a sectional view of an electro-optical transducing tube in accordance with this invention;

Fig. 2 is an enlarged plan view taken along the line 2--2 of Figure l; and

Fig. 3 is a sectional. view of Fig. 2.

The invention finds its primary utility in television pickup, or camera tubes, of the type having a photoconductive target electrode. Pickup, of camera tubes of this j 2,900,569 Patented Aug. 18, 1959 type generally comprise an evacuated bulb within which is mounted an electron gun of any well known construction for developing a deectable scanning beam of electrons usually referred to as a cathode-ray beam. The beam is focused on, and deliected over, a photoconductive transducing target to constitute a raster of well deiined size and shape, usually square or rectangular, i.e. having a conventional four-by three aspect ratio. The target is usually supported on a light-transparent backing, which for example may constitute :the end wall or face plate of the glass bulb. Such a target usually consists of a light transparent layer or lm of high electric conductivity which is coated or otherwise applied to the substantially circular, inner surface ofthe glass support facing the electron gun, and then a layer of photoconductive material, such for example as antimony tri-sulphide, is applied over the conductive layer. The photoconductive material is substantially an insulator when not exposed to light rays. The target hasrthe property of coacting mutually with the scanning beam and with the light intensities existent on each successive elemental area of the target so as to cause the elemental areas to change their conductivity and thereby produce signal voltages under control of the scanning beam.

The photoconductive layer of the target, whether it be antimony tri-sulphide, or any other photoconductive material, being a non-conductor when not exposed to light, nevertheless has its conductivity at each elemental area thereof variable in accordance with the variations of the light intensity to which each such area is exposed. In other words, the conductivity of the photoconductive layer is proportional to the amount of light exciting it and the variation is limited to the particular areas which are so excited. Thus, when the surface of such material is subjected to light from a scene or subject to be televised, the various elemental areas of the target change their normal non-conductance condition to correspond respectively with the light intensities from respective elemental areas of the scene or subject being televised, it being understood, of course, that these light rays are suitably projected or focused on the target.

Usually the transparent conductive backing or signal plate on which the photoconductive material is applied, is biased to a steady positive target potential, e.g. from 10 to 200 volts, with respect to the cathode of the electron gun which scans the target. In eifect, the conductive signal plate and the photoconductive coating thereon constitute a charge-storing condenser with the charge at each elemental area being a function of the incident light between scannings by the electron beam. The signal currents, or impulses, are produced by subjecting the condenser to a discharging action by the scanning beam which restores each elemental area that it strikes to a predetermined datum potential for example cathode or nearcathode potential. It is clear, therefore, that if any elemental areas of the target have received a charge, either by dark conductivity or by exposure to light, and have not been appropriately discharged by the scanning beam, they tend to assume an undesired steady potential. I have found that this effect tends to produce unstable electric signal output characteristics, especially where the area that is not properly discharged is not located within the boundaries of the actual scanning raster.

Usually, the light transparent conductive coating or signal plate is applied over substantially lthe entire end wall or face plate of the tube so as to make complete peripheral contact with a separate conductive ring sealed through the tube wall and by means of which the positive biasing potential can be applied thereto; and usually the photoconductive material is substantially coextensive with the conductive signal plate. Since the face plate is usually circular and since the raster is usually square 3 or rectangular, there alwaysexists a region surrounding the raster which is not subjected to the discharge or cathode stabilization by the cathode-ray beam.

I vhave -found that this area beyond the boundariesof the raster, after a period of operation -of the tube,acquires a positive potential which is substantially demarcated in level from the potential level of the scanned raster area. The scanned raster area over a period of repeated scannings `can be considered as having ground or near-ground potential. In other words, there is a well defined potential step between the scanned and nonscanned areas of the photoconductive layer. I have further found that the area of undesired positive potential beyond the boundaries of the raster tends to spread its effect into the raster area. The net result is that the edges of the picture reproduced under control ofthe signals from such a conventional pickup tub' target,'tend to show crawL ripple or other instability. In extreme cases the cathode-ray beam is actually unable properly to discharge the target adjacent the edges of the raster so that the ytarget becomes inoperative as a proper transducing agent at the edges of the raster.

The present invention, therefore, has for one of its principal objects, overcoming of the above noted and other disadvantages. Referring to the drawing, the tube may comprise an evacuated glass bulb, or envelope, which has suitably supported at one end therein an electron Vgun 11 of any well' known construction comprising, for example, an electron emitting cathode 12, a control electrode 20 and one or more accelerating electrodes 22 for developing an electron beam which can be focused in an elemental spot upon the light transducing target electrode 13 mounted within the opposite end of the bulb. This target electrode comprises a light transparent backing iilm 14 of a good electrical conductor such, for example, as a tin compound like tin oxide or tin chloride. The conductive film 14 can be applied in any suitable manner, for example as a coating, directly on the inner surface of the light transparent end wall or face plate 15 of the glass bulb.

In accordance with the invention, the conductive light transparent lm 14, which constitutes the signal plate, does not completely cover the end wall or face plate of the tube but is of square or rectangular shape and provided with at least one laterally extending portion 16 for making Contact with a metal ring 17 sealed into and through the wall of the glass bulb. It will be understood, of course, that the metal ring 17 should be of a suitable metal or alloy which has substantially the same coeil'- cient of expansion as the glass of the bulb 10 and of the face plate 15 so'as to preserve a vacuum tight seal. If desired, one or more similar narrow strip extensionsrl of the lm 14 may be provided to increase the contact area between the ring 17 and the signal plate 14. The size and shape of the raster 19 scanned by the electron beam are indicated in Figure 2 by the dot-dash outlfne.

In accordance with another feature of the invention, the size of the rectangular or square conductive signal plate or iilm 14 is only slightly less than the actual size and shape of the scanning raster 19.

After the conductive iilm 14, with its integral connection strips 16 and 18, has been applied to the face plate 15, a layer of photoconductive material23 is applied over substantially the entire inner surface of the face plate 15. The invention is not limited to any particular manner of applying the lilm 14 and the photoconductive material 23. For example, the photoconductive material 23 may be applied by any of the known evaporation techniques. Of course other methods of applying the materials may be utilized in accordance with this invention. The layer 14 may be any transparent, conductiveV material, e.g. tin chloride, and' may be applied by any of the known techniques. After the materials 14 and 23 have been applied, the tube, is subjected to the usual .i 2,900,569V f jr evacuation and other techniques well known in the elecftron tube manufacturing art.

When the tube is to operate, for example, with a low velocity electron beam, the ring 17 can be connected through a suitable load resistor 21 to a source of positive direct current potential, whereby film 14 is biased to as much as 200 volts positive with respect to cathode 12 whichV may be grounded. The usual conductive Wall coating, or nal `accelerating electrode 24, can 'be connected to a source of positive potential, for example positive 250 or 300 volts, as is the final anode of the gun 11. The electron beam, therefore, deposits upon each elemental area of the target suiicientv electrons to neutralize the accumulated positive charge at each area resulting from its light excitation, and in doing so there are generated the proper video output signals across resistor 21V which can be coupled to any well known video amplilier (not shown).

Y, over said conductive layer and facing said means, and said photoconductive layer extending beyond the *boum* l have found that, with the substantial correlation between the size and shape of the signal plate :14V and the actual scanning raster 19, the above notedV instability at the edge of the raster is avoided and the tube is stable in its output characteristics. i

lt will be understood, of course, that the invention is not limited to the particular materials mentioned forthe conducting iilm 14 and the photoconductive material 23. Other photoconductive materials, such as cadmium-sulphide or selenium, may be employed.

While the invention has been described in connection with a square or rectangular signal plate 14 and a corresponding raster, it will be understood that if the raster is of any other shape, for example circular, then likewise the signal plate 14 should also be circular and of substantially the same size and shape as the raster. thermore, while reference has been made to the operation of the tube with aV scanning beam of low electron velocity, it is believed that the invention also can be used with the tube operating with a scanning beam of high'Y electron velocity, which causes the target to emit sec ondary electrons when struck by the scanning beam, in which event a suitable secondary electron collector grid may be mounted adjacent to the target on the side facing the gun, as is well known in the art. It will be understood, of course, that any well known means may be provided for focusing the beam from the electron gun in an elemental spot upon the target 13, and that any Well known means may be employed for subjecting the beam to the desired scanning pattern to cover the desired raster.

. Various changes and modifications may Vbe made in the disclosure without departing from the spirit and scope of the invention.

What is claimed is:

1. Electron tube apparatus comprising, a tube having means to develop a beam of electrons, a light-responsive` target arranged to be scanned by said beam ina scanning raster for developing output electric signals correspond# ing to the light excitation of the scanned areas of the target, said target including a conductive layer of a predetermined size, means for making an electrical connection with said conductive layer, a photoconductive layer daries of said conductive layer.

2. Electron tube apparatus comprising, a tube having an electron gun for developing Va beam of electrons, a light-responsive layer of photoconductive material arranged to be scanned by said beam in a rasterof predetermined shape and size todevelop output electric sig-Y nals corresponding to the light excitation of the scanned areas of the target, saidhtarget comprisingV a layer of photoconductive material'of the kind which is normally a non-conductor when not exposed to light but which becomes a conductor of variable conductivity corresponding to the variation of light thereon, said layer extending substantially beyond the boundaries of said raster, a light-transparent metal lm in direct Contact with said layer on the side of said layer remote from the gun, said ihn having a size slightly less than said raster, and lead-in means to apply a biasing potential to said lm.

3. Electron tube apparatus comprising, an evacuated envelope having an electron gun to develop a beam of electrons, a light-transparent support, a light-responsive target of the photoconductor kind carried by said support and arranged to be scanned in a raster of a predetermined size, an electrically conducting signal plate atfrached to said support and having a size which is slightly smaller than the size of said raster, and a coating of photoconductive material covering said signal plate on the side thereof facing the gun and substantially covering the surface of said support beyond the boundaries of said signal plate.

4. An electron discharge device especially designed for transducing light signals into corresponding electric signals, comprising an evacuated bulb having a conductive lm coated on a wall of the tube, a coating of photoconductive material on said film, said conductive lm being located substantially within the boundaries of said coating to leave a portion of said coating which is unbacked by said film, a conductive ring surrounding said coating and electrically connected to said conductive lrn, an electron gun, and means for applying potentials to said device for scanning said layer with electrons from said gun in a raster, said raster being slightly larger in size than said conductive film.

5. A pickup tube comprising an evacuated bulb having a face plate at one end, a layer of photoconductive material substantially coextensive with said face plate, a light-transparent signal plate sandwiched between said layer of photoconductive material and said face plate and having a surface extent which is substantially less than said face plate, and an electron gun for scanning said layer of photoconductive material in a predetermined raster, said raster being slightly larger than said signal plate.

6. A pickup tube comprising an envelope, means for producing an electron beam in one end of said envelope, a target electrode in the other end of said envelope and in the path of said beam, means for scanning said beam over said target in a raster pattern, said target including a photoconductive layer substantially larger than said raster pattern and a signal plate, said signal plate being slightly smaller than said raster pattern whereby charges at the edges of said raster are discharged by said beam.

7. An image transducing tube of the kind described comprising, an evacuated bulb, a light-transparent support, a photoconductive target attached to said support, said targetk including a layer of normally non-conducting photoconductive material, a light-transparent conductive signal plate sandwiched between said layer and said support, said plate having a surface extent which is substantially less than that of said layer, an electron gun for scanning said target with an electron beam in a raster which is only slightly larger in surface extent than said signal plate but is substantially smaller in surface extent than said layer, and lead-in means between said support and said photoconductive material for applying a biasing potential to said signal plate.

References Cited in the le of this patent UNTED STATES PATENTS 2,368,884 Shade Feb. 6, 1945 2,654,852 Goodrich Oct. 6, 1953 2,687,484 Weimer Aug. 24, 1954

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